Electrophotographic printing is a well known method of producing documents. Electrophotographic printing is typically performed by exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto the latent image to form a toner image. That toner image is then transferred from the photoreceptor, either directly or after an intermediate transfer step, onto a substrate such as a sheet of paper. The transferred toner image is then permanently fused to the substrate using heat and/or pressure, thus producing the desired document. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the creation of another image.
The printing process described above can be modified to produce color images. One color printing process, which is sometimes referred to as the multipass intermediate belt process, develops an image as described above using a first color of toner, transfers the resulting toner layer onto an intermediate belt, develops a second toner layer of a different color, transfers that second toner layer onto the intermediate belt in superimposed registration with the first transferred toner layer, and then repeats the develop-transfer process for third and fourth toner layers of third and fourth colors. After the composite toner image is formed on the intermediate belt the image is transferred and fused onto a substrate. In the multipass intermediate belt process the development of each color of toner layer is essentially independent of the development of the other colors of toner layers. This is beneficial since the developing stations can be set up to produce the desired target toner masses for each color of toner independent of the other developing stations.
Another color electrophotographic printing process, referred to as either image-on-image processing or REaD (Recharge, Expose, and Develop), superimposes toner images of different colors of toner on the photoreceptor prior to the transfer of the composite toner image onto the substrate. While this process is beneficial, it has several drawbacks. For example, when recharging the photo receptor in preparation for creating a color toner image after a previous toner image has been developed it is important to level the voltages between the previously toned and the untoned areas of the photoreceptor. Although it may be possible to achieve voltage uniformity by simply recharging previously toned layers to the same voltage level as neighboring untoned areas, an effect referred to as residual toner voltage complicates the process.
Residual toner voltage is the voltage difference that occurs between toned areas which have been re-exposed and untoned areas which have been exposed. Residual toner voltage reduces the effective development field in the toned areas relative to the untoned exposed areas, thereby hindering the attempt to achieve a uniformly consistent developed mass of the subsequently deposited toner image. The problem becomes increasingly severe as additional toner images are developed. Color quality is threatened since the residual toner voltage can cause color shifts, increased moire effects, increased color shift sensitivity to image registration, and toner spreading at image edges.
Various solutions to the problems caused by residual toner voltage have been proposed. For example, a co-pending U.S. patent application entitled "Method and Apparatus for Reducing Residual Toner Voltage," Ser. No. 08/347,616 discloses a recharging method and apparatus which uses photoreceptors with highly sloped output current (current applied to the charge retentive surface) verses photoreceptor surface voltage characteristics. However, that system's reduction in residual toner voltage is rather limited.
A recharging method which reduces photoreceptor voltage distribution nonuniformities is described in Japanese Patent application No. Hei 1-340663, Application date Dec. 29, 1989, Publication date Sep. 4, 1991, assigned to Matsushita Denki Sangyo K.K. That reference discloses a color imaging system which uses two rechargers. The first recharger applies a voltage to the photoreceptor which is higher than the voltage the photoreceptor is to have when it passes to an exposure station. The second recharger reduces the surface voltage of the photoreceptor to that which the photoreceptor is to have when it passes to the exposure station. However, patent application No. Hei 1-340663 teaches that the difference in voltage between those applied by the first and second rechargers is sufficient to insure that the polarity of all toner is reversed after passing through the rechargers. The net result is a reduction in the residual charge in the toned areas and a reduction in toner spray. Toner spray is a phenomena that occurs when a photoreceptor carrying a toner image is recharged to a relatively high charge level and then exposed. In areas where the edges of prior developed images align but do not overlap with the edges of a subsequent image, the toner of the prior image tends to spray or spread into the subsequently exposed areas (which have a relatively lower charge level). Reversing the polarity of the toner prevents toner spray since the reversed polarity toner is not attracted to the exposed areas.
While the method described in Japanese Patent application No. Hei 1-340663 is effective in reducing residual toner charge and toner spray, when a composite toner image comprised of a substantial amount of toner is reversed in polarity, a different problem can develop. After recharging and subsequent exposure, the toner in the prior developed toner image has a polarity which is opposite that of both the background untoned areas and the incoming toner which is to form a toner image. An interaction occurs among the three distinctly charged regions. For example, in a system having a negatively charged photoreceptor and which uses discharged area development (DAD), the negatively charged toner used for development would be reversed in polarity after recharge using the teachings of Japanese Patent application No. Hei 1-340663. The positively charged toner layer would then be attracted to the negatively charged background areas and the incoming negatively charged toner. The positively charged toner then tends to splatter onto neighboring bare background regions. This occurrence is called the "under color splatter" defect (UCS). UCS causes unwanted blending of colors and spreading of colors from image edges onto background areas. Furthermore, a relatively large voltage difference between the first and second rechargers would cause a significant amount of stress to be applied to the photoreceptor. That stress could reduce both the image quality and the life expectancy of the developer material.
Co-pending and commonly assigned U.S. patent application,"Split Recharge Method and Apparatus for Color Image Formation," Ser. No. 08/347,617 discloses a recharging method which attempts to solve the UCS problem. Specifically, U.S. patent application Ser. No. 08/347,617 discloses a split recharge configuration wherein a first corona generating device recharges a charge retentive surface having a developed image thereon to a higher absolute potential than a predetermined potential, and then a second corona generating device having an AC voltage supplied thereto recharges the surface to the predetermined potential. The difference in the photoreceptor surface potential after being recharged by the first corona recharge device and the second corona recharge device is called the "voltage split." Significantly, the alternating current from the second recharger substantially neutralizes the electrical charge associated with the image. To prevent the toner in the prior developed toner powder image from reversing polarity the mount of voltage split is limited. This limits the mount of residual toner voltage reduction that can be achieved.
While the above methods are all useful for reducing residual toner voltages they do not completely eliminate the problem. Since image-on-image color electrophotographic processes are sensitive to the effects of residual toner voltages methods of compensating for residual toner voltages would be highly desirable.